In a liquid crystal display, various kinds of optical films, such as a polarization film and a retardation film, are used. In a polarization film of a polarizing plate for use in a liquid crystal display, a cellulose ester film is laminated as a protection film on one side or both sides of a polarizer composed of a stretched polyvinyl alcohol film. A retardation film is used for the purposes, such as expansion of a view angle and improvement in contrast, and the retardation film is produced by stretching films, such as polycarbonate, cyclic polyolefin resin and cellulose ester, so as to provide retardation. The retardation film may also be called an optical compensation film.
In a plasma display and an organic EL display, various kinds of functional films, such as an antireflection film and a protection film are also used.
The optical films used for these display devices are required to satisfy the following points that there is no optical defect, retardation is uniform, and, especially, there is no dispersion in a phase axis. In particular, since a monitor or a TV become larger and higher definition, the above requirements for qualities becomes more severe.
The producing method of an optical film is roughly divided into a melt casting film producing method and a solution casting film producing method. In the former, a film is produced in such a way that polymer is heated and melted to form a molten material (melt), the melt is cast on a support, cooled and solidified to form a film, and the film is stretched, if needed. In the latter, a film is produced in such a way that polymer is dissolved in a solvent to form a solution (dope), the solution is cast on a support to form a film while evaporating the solvent, and the film is stretched, if needed.
In any one of the film producing methods, polymer is solidified on a support to form a polymer film and the polymer film is separated from the support. Thereafter, the polymer film is subjected to processes of drying, stretching and the like while being conveyed by the use of a plurality of conveying rollers.
As compared with a solution casting film producing method which uses a solvent in large quantity, since a melt casting film forming method does not use a solvent, it can expect to increase productivity. Therefore, the melt casting film producing method is more desirable from the above-mentioned viewpoints. However, the melt casting film producing method has the following drawbacks. Resin, additives, etc., which have decomposed thermally at the time of the film production, adhere to conveying rollers to cause the pollution of rollers. If the pollution advances, the pollution is transferred onto a film to cause spotty unevenness and concavo-convex on the film. As a result, the quality of a film becomes deteriorated. Further, when rollers become dirty, a film production is obliged to be interrupted due to the cleaning of the rollers. Therefore, the development of a roller cleaning method in order to keep continuous production has been an important theme.
With regard to the roller cleaning method, Patent Documents 1 to 3 propose the following methods.
In a producing method and apparatus of resin covered (lamination) paper, in particular, in a laminating apparatus including a process of covering with a molten resin, Patent Document 1 discloses a cooling roller cleaning method to remove low molecule components adhering to a cooling roller. As a cleaning method, Patent Document 1 discloses a method of applying energy on the surface of a cooling roller by the use of a high output laser light source or flame of a flame burner.
Patent document 2 discloses a method of removing materials adhering on a roll surface by irradiating ultraviolet rays onto the roll surface used for the production of a film.
In order to reduce flaws on the surface of film generated in a film producing process of a thermoplastic resin film and to clean dirt adhering on a cooling roller, Patent document 3 discloses a method of removing organic substances adhering on a cooling roller by irradiating plasma to the cooling roller which comes in contact with film being conveyed.
Patent Document 1: Japanese Patent Unexamined Publication No. 2002-240125
Patent Document 2: Japanese Patent Unexamined Publication No. 2003-89142
Patent document 3: Japanese Patent Unexamined Publication No. 2001-62911